1. Field of the Invention
The invention generally relates to regeneration of cryogenic vacuum pumps. More particularly, the invention relates to a method for reducing particulate generation from regeneration of cryogenic vacuum pumps.
2. Background of the Related Art
Cryogenic vacuum pumps (cryopumps) are widely used in high vacuum applications. Cryopumps are based on the principle of removing gases from a vacuum chamber by binding the gases on cold surfaces inside the cryopump. Cryocondensation and cryosorption are the main mechanisms involved in the operation of the cryopump. In cryocondensation, gas molecules are condensed on previously condensed gas molecules, and thick layers of condensation can be formed, thereby, pumping large quantities of gas. Cryosorption is commonly used to pump gases that are difficult to condense at the normal operating temperatures of the cryopump. In this case, a sorbent material, such as activated charcoal, is attached to the coldest surface in the cryopump, typically a second stage of a cryoarray. Because the binding energy between a gas particle and the adsorbing surface is greater than the binding energy between the gas particles themselves, the gas particles that cannot be condensed are removed from the vacuum system by adhering to the sorbent material. However, the effect of the adsorbing surface diminishes as the gas particles are adsorbed by the adsorbing surface of the sorbent material. After several monolayers of adsorbed gas particles have built up over the adsorbing surface, the adsorbing surface stops adsorbing the gas particles by cryosorption unless the adsorbing surfaces are regenerated or restored to a fresh, operable state.
Cryopumps typically include two stages of cryoarrays. A two-stage cryopump includes a first stage cryoarray, which typically operates at temperatures between about 50 K and about 100 K, and a second stage cryoarray, which typically operates at temperatures between about 10 K and about 20 K. The two-stage cryopump is typically matched to a closed-loop helium refrigerator that includes a two-stage expander which creates cryogenic refrigeration by the controlled expansion of compressed helium. Each stage of the cryoarrays is thermally connected to and independently cooled by one matching stage of the expander.
Different gases are pumped on different cryoarray surfaces within the cryopump. The first stage cryoarray typically pumps gases, such as water vapor and carbon dioxide, at relatively high temperatures by cryocondensation. An outer surface of the second stage cryoarray typically pumps gases, such as nitrogen, oxygen and argon, at the normal operating temperature of the second stage. An inner surface of the second stage cryoarray is typically coated with a sorbent material that pumps the noncondensable gases, such as hydrogen, neon and helium, by cryosorption. The sorbent material typically comprises charcoal and is bonded, glued or otherwise attached to the second stage cryoarray.
Under normal operating pressures, conditions of molecular flow exist in the cryopump. Practically all molecules entering the pump will strike the first stage cryoarray and the outer surface of the second stage cryoarray before reaching the sorbent material on the inner surface of the second stage cryoarray. Thus, all gases except the noncondensable gases, such as hydrogen, neon and helium, are pumped by cryocondensation before reaching the sorbent material, leaving the inner surface of the second stage free to pump the noncondensable gases by cryosorption.
Finite amounts of gas can be accumulated on the pump surfaces before performance deteriorates and eventually becomes unacceptable. Particularly for the second stage cryoarray, when several monolayers of adsorbed gas have been built up, the sorbent material loses its adsorption abilities, and the noncondensable gases can no longer be pumped by cryosorption on the sorbent material. At this point, captured gases on the cryoarrays need to be released and expelled from the cryopump, thereby renewing the pumping surfaces for further service. This process, called regeneration, includes heating the cryopump until the captured gases evaporate. The released gases are then removed from the cryopump through a pressure relief valve and/or are removed by a roughing pump that is attached to the cryopump. The cryopump is then cooled to its operating temperature, and normal cryopump operation is resumed.
A standard method for removing all captured gases, including condensed water vapor, heats the cryopump to a regeneration temperature while purging the cryopump with a regeneration or purge gas, typically an inert gas. The cryopump is typically purged for some time after reaching regeneration temperature, typically the same as the temperature of the regeneration gas, and is pumped with a roughing pump to remove the gases in the cryopump. Since all captured gases are removed from the cryopump, including both the first and second stage cryoarrays, this process is called full regeneration. Full regeneration typically requires several hours to complete. During this time, the cryopump and the equipment to which it is attached are inoperable, resulting in costly downtime for the system.
To shorten regeneration time, a process called partial regeneration or fast regeneration has been developed. In partial regeneration, only the gases pumped by the second stage cryoarray are removed from the cryopump. Typically, the second stage cryoarray is heated to a temperature between about 110 K and about 160 K, preferably about 125 K, by flowing a regeneration gas, typically an inert gas such as dry nitrogen, into the cryopump and/or by activating a heater that is thermally attached to the second stage cryoarray. However, the refrigerator continues to cool the first stage cryoarray to prevent release of gases from the first stage cryoarray. The released gases from the second stage cryoarray are removed using a roughing pump that is attached to the cryopump. Because only the second stage cryoarray is heated and regenerated, the time required for cryopump regeneration is decreased significantly.
A particular problem encountered in both full regeneration and partial regeneration of the cryopump is that the cryopump experiences thermal and mechanical shock at the beginning of the regeneration cycle caused by introducing the regeneration gas into the cryopump and heating the cryoarrays. More specifically, the cryopump experiences a pressure burst at the beginning of the regeneration cycle because of the initial introduction of the regeneration gas into the cryopump and the release of the gases from the cryoarrays. The pressure burst is typically caused by the uncontrolled introduction of a purge or regeneration gas at a high pressure (typically at about 80 PSI) into the cryopump, and the pressure burst has been observed on a strip chart recorder as a fast, nearly instantaneous pressure increase in the cryopump. The pressure burst causes fracturing of the cryoarray material and particulate generation from broken pieces of the cryoarray material, such as flaking and shedding of the charcoal. The particulates dislodged from the cryoarray material lead to contamination of the vacuum processing chamber, and the contamination of the vacuum processing chamber causes defect formations on substrates subsequently processed in the chamber. The sudden increase in temperature of the cryoarrays also contributes to fractures of the cryoarray material and particulate generation from broken pieces of the cryoarray material, which leads to contamination of the vacuum processing chamber and defect formations on substrates subsequently processed in the chamber.
Therefore, there is a need for a method of regenerating a cryogenic vacuum pump that significantly reduces the particulate generation from the cryoarray material caused by the thermal and mechanical shock experienced by the cryopump during regeneration. Particularly, there is a need for a regeneration method that significantly reduces or eliminates the pressure burst that occurs at the beginning of the regeneration cycle. Also, there is a need to control the temperature ramp rate of the cryoarrays to reduce thermally induced stress on the cryoarrays during the regeneration cycle.